The invention relates to an apparatus for producing a plasma and for the treatment of substrates therein. The apparatus includes a microwave generator, a gas containing chamber in which the substrate is coated, and a magnet system for the production of local electron-cyclotron resonances.
In numerous fields of technology it is necessary to apply very thin coatings of pure substances to certain objects. An example is window glass which is provided with a thin coating of metal or metal oxide in order to filter certain wavelength ranges out of sunlight. In semiconductor technology, thin coatings of one or more substances are often applied to a substrate. It is especially important that the thin coatings not only be pure, but also that they be precisely measured out so that the coating thicknesses--and, in the case of coatings of chemical compounds, their composition--will be accurately repeatable. These coating thicknesses are, as a rule, between two and several thousands of nanometers.
A variety of methods are known for applying thin coatings to films, glass and other substrates. In a first method, the thin coating is applied by chemical or electrochemical deposition, while in a second method the coating is applied by evaporation in a vacuum. With evaporation it is difficult to provide large areas with very thin coatings with the required uniform precision and repeatability, and consequently a third method, known as the sputtering or cathode spraying process, is used. For the deposition of a thin coating from the gas phase, sputtering is, of course, unsuitable.
To be able to deposit a pure substance or a chemical compound from the gaseous phase, the substance or compound is converted to the plasma state. The radicals formed in the plasma deposit themselves on the substrate. For the production of such a plasma, different forms of electrical energy can serve. For example it is possible to use direct currents, low-frequency alternating currents or corona discharges for the production of plasmas. Especially advantageous is the production of plasma by microwaves, because in this case no electrodes are needed, which can contaminate and become ablated, and because the plasma produced by microwaves has a greater density of ions and radicals and therefore can be kept at a higher pressure than the plasma produced by other methods. Furthermore, the chemical structure of starting monomers can be preserved at least partially. Lastly, the microwave plasma is also favored for the establishment of cold cathode ion sources.
It is true that usually only small volumes of plasma can be produced by microwaves, because the apparatus by which the microwave energy is delivered to the plasma--e.g., antennas, waveguides and cavity resonators--do not permit the production of large volumes of plasma. To produce a gas plasma, the delivered electrical field strength must exceed the electrical breakdown field strength of the gas. Since the breakdown field strength increases with increasing gas pressure, high electrical field strengths are necessary at high pressures.
An apparatus for the production of plasmas by means of electromagnetic radiation is known, with which high field strengths are produced (U.S. Pat. No. 3,814,983).
In this apparatus a delay line, i.e., a microwave conductor of low group velocity ("slow wave structure") is used for the purpose of feeding the electrical energy to the plasma, the energy source being located outside of the receptacle and its electrical field passing through the receptacle wall. This delay line consists of a "semiradiating" system about 90 cm long, which operates in the degenerate .pi./2 mode or close to the degenerate .pi./2 mode. Operation in the vicinity of the band edge, i.e., either in the degenerate .pi./2 mode or in the .pi. mode, leads to especially strong electrical fields in the vicinity of the delay line. The reason for this lies in the circumstance that the electrical field strength is inversely proportional to the group velocity of the wave, which in the vicinity of the edge of the band assumes a very small value. Furthermore, in this system the electrical field strength decreases with the distance perpendicular to the plane of the delay line. It is true that with this apparatus no large-volume plasmas with a very large, uniform plasma zone can be produced. It follows that the rate of deposit of polymers is irregular across the entire substrate width in the known apparatus. Moreover, interactions take place between the waves, which occur in the delay line, in the window dielectric and in the plasma; i.e., poorly understood interferences develop, which adversely affect the configuration of the plasma zone.
To equalize the rate of deposition in the case of polymers it has already been proposed, in an apparatus according to U.S. Pat. No. 3,814,983, that, in addition to the known delay line, at least a second elongated delay line be disposed on the same side of the substrate (German Federal Pat. No. 31 47 986). But this "crossed structures" arrangement has the disadvantage that the strongest plasma burns directly at the inside of the microwave window where the microwave is injected, and this results in an especially great and undesirable coating of this window.
Furthermore, an apparatus is known whereby a plasma is produced by means of a high-frequency wave which is injected into a waveguide in which a glass tube is situated in which the plasma is produced (German Federal OS No. 31 44 016), to which U.S. Pat. No. 4,438,368 corresponds. Around the plasma producing tube there is in this case provided a coil which produces a magnetic field along the axis of the glass tube. At a circuit frequency .omega. of the high-frequency field, and a magnetic flux density B, the electron-cyclotron resonance frequency will be .omega.=e.times.B/m. At this resonance frequency the coupling of the high-frequency wave to the plasma electrons is especially strong. It is a disadvantage even in this known device, however, that only relatively small plasma zones can be produced. Furthermore, the glass tube easily takes on coatings deposited from the gas phase.
It is therefore the object of the invention to create an apparatus whereby it will be possible on the one hand to produce a uniform, large-volume plasma, and on the other hand to keep the plasma away from the microwave window.